Many fluids and other materials used in science and industry, especially biological materials, contain particles having various sizes. For example, blood contains red blood cells, white blood cells, platelets, and other components which can be distinguished by their size. Further, a wide variety of biological materials, such as cells, proteins, antibodies, and DNA, can be bound to polystyrene and latex beads of various sizes. For example, antibodies immobilized on beads of various sizes are used in integrated protein bioassays, an accepted tool of clinical medicine. Thus, a system that can separate particles by size can enable red blood cell counts, bead-based assays, and other measurements useful in medicine, and in other applications in science and industry.
A variety of techniques have been developed to separate particles by size, such as electro-osmotic flow, magnetic separations, dielectrophoretic separations, electrophoretic separations, diffusion-based separations, cantilever-binding mass-sensing assays, and acceleration techniques. However, the practical value of these prior techniques may be limited by real-world considerations such as cost, power or voltage requirements, sample volume requirements, difficulty of manufacture, complex construction, moving parts, measurement quality, reliability, and response time.
For example, a typical electro-osmotic microfluidic assay system for separating particles of varying sizes utilizes the differential mobility of the differently-sized particles in a high electric field, but this technique may require a conducting medium and high voltages. Magnetic bead separation may require the use of high currents for sufficient magnetic field production, or physical movement of external magnetic components, which may be more costly, more difficult to manufacture, and less reliable than techniques which do not require moving parts. Dielectrophoretic and electrophoretic separations both may require electrical connections to the fluid as well as a non-conducting medium and high voltages. Binding to a cantilever and detecting the resulting mass or strain change may require coating of the cantilevers with specific binding molecules, making the test instrument specific to a particular test. Thus, each of the prior art techniques for separating particles of differing sizes may have disadvantages that limit practical utility.
Particle manipulation with a projected acoustic field has been investigated by many researchers. Acoustic radiation forces generated by concave transducers have been used to concentrate particles at pressure nodes. See, for example, U.S. Pat. No. 6,332,541 to Coakley et al., which discloses an apparatus for separating particles from a fluid. However, separation of particles by size using acoustic radiation force has not been demonstrated.